The instant invention is concerned with a method for increasing the recovery of hydrocarbon products from formations containing viscous petroleum by improving the vertical conformance of said heavy oil formations. In particular, the method of this invention provides a technique for recovery of viscous hydrocarbons from subterranean formations by selectively treating intervals of the formation for purposes of establishing spaced regions for heat treatment within the formation.
Successful production of hydrocarbon products from formations containing viscous hydrocarbons cannot be undertaken by conventional methods. Formations containing viscous hydrocarbons and, in particular, formations containing tar sand or bituminous sands are known to exist in Canada, Western United States, Venezuela and, to some extent, in Europe and Asia. One such formation is the Athabasca tar sand deposit located in the Province of Alberta, Canada.
Typically, tar sand deposits consist of fine quartz sand coated with water and saturated with a very viscous hydrocarbon. The remainder of the space in the deposit may be filled with connate water and, in some deposits, may contain small amounts of air or methane.
Generally, the grains of sand comprise approximately 80 to 85 percent weight of the entire tar sand deposit and this corresponds to a void volume of approximately 35 percent.
Viscous hydrocarbons and water comprise approximately 15 to 20 percent by weight of the tar sand deposit. The quantity of viscous hydrocarbon may vary from about two percent to about 16 percent. The gravity of the viscous hydrocarbon in tar sand formation may range from about 6.degree. to about 8.degree. API and the specific gravity of this material at 60.degree. F. is from about 1.006 to about 1.027.
Strip mining is one method which may be employed to recover viscous hydrocarbons contained in tar sand deposits. However, this method of recovery is economically feasible only when formations to be produced are relatively close to the surface, i.e., when the ratio of the overburden thickness to the tar sand deposit thickness is about 1 or less. In many cases when the tar sands are located at greater depths, and this ratio is greater, other economical means of recovery must be utilized. In such recovery techniques well bores are drilled into the formation. Some of these wells are treatment or injection wells in which fluids may be injected to push or sweep the viscous hydrocarbons to another production well bore from which it is recovered. Alternatively, the treatment fluids may be introduced into a formation with the purpose of causing the viscous hydrocarbons to flow into the same well bore from which they may be recovered.
Generally, viscous hydrocarbons are found in two types of formations, referred to as homogeneous and nonhomogeneous formations. In a homogeneous formation, the percentage of hydrocarbon or petroleum product is very uniform over the entire formation. The difference in the overall hydrocarbon content of different intervals or regions of the formation may vary by about 2 to about 5 percent in such formations. In contrast, nonhomogeneous formations may contain sands that have a wider range of oil saturation.
In nonhomogeneous formations, because of the variation in hydrocarbon content in the various intervals or zones, the formation permeability is not constant. Even in homogeneous formations the permeability may vary, but usually not to the same extent as in nonhomogeneous formations.
In viscous petroleum formations, and particularly in tar sand formations, a successful in situ method for recovery of hydrocarbons frequently involves reducing the viscosity of the hydrocarbons or petroleum in the formation by application of heat to the formation. Upon application of heat, the hydrocarbons become flowable whereupon the hydrocarbons will flow or may be driven out of the formation to a production well from which the hydrocarbons may be recovered. Heat is applied to the formation frequently by injecting a heated fluid, e.g., steam, into the formation to raise the formation temperature. Alternatively an in situ combustion may be initiated to apply heat to the viscous hydrocarbons and improve flowability. In a formation of varying permeabilities, however, the steam will typically invade those areas of high permeability successfully applying heat and reducing the viscosity of the hydrocarbons in the environs of the high permeability regions. In some viscous petroleum formations and tar sand formations, steam will actually form an emulsion barrier at the point where it encounters the formation thus precluding the invasion of steam to large areas of the formation and inhibiting successful recovery.
One condition encountered in steam flooding of viscous petroleum formations has been termed "steam override." In this condition the steam introduced via an injection well does not significantly penetrate the formation at its lower level; rather, the steam has more of a tendency to move upwardly and across the upper level of the formation to the production well. By following such a path the steam serves to reduce the viscosity of only a small amount of the valuable hydrocarbon in the formation and recovery is poor.
The prior art has accordingly attempted to solve these problems by pretreating the formation prior to the application of heat. For example, low molecular weight hydrocarbons such as propane or pentane and the like are injected into the formation to partially dissolve the viscous hydrocarbon or bitumen and create a region more readily permeated by the heated fluid. In these applications, however, the injected hydrocarbon solvent will again tend to flow through the formation by taking the path of least resistance. Accordingly, permeability is enhanced in those regions that originally were most permeable. As can be readily appreciated, such a condition will exist to a significant extent in nonhomogeneous formations, although the permeability gradient in homogeneous formations can be sufficiently significant to leave a large portion of the hydrocarbon or bitumen unswept from the formation following the heat treatment.
Many methods for the recovery of hydrocarbons have been described including water flooding, gas pressurization, in situ combustion and miscible flooding. For example, U.S. Pat. No. 2,862,558 discloses a method for the recovery of hydrocarbons from tar sands which comprises treating the tar sand formation with a mixture of steam and kerosene or a liquid hydrocarbon. U.S. Pat. No. 3,354,958 discloses a two step method for the recovery of oil which comprises first injecting steam and then injecting a volatile hydrocarbon. U.S. Pat. No. 3,608,638 describes a method for the recovery of heavy oil from tar sand formations which comprises injecting a heated hydrocarbon solvent into the upper portion of the deposit and heating the tar sand oil below said upper level.
Other patents such as U.S. Pat. Nos. 3,954,141, 4,004,636 and 4,007,785 disclose methods for the recovery of petroleum from viscous petroleum-containing formation by utilizing a multiple solvent injection into the formation. In particular U.S. Pat. No. 4,004,636 combines a multiple solvent treatment of the formation with a thermal treatment of the formation to recover heavy oils.
Another patent, U.S. Pat. No. 3,147,803, discloses a secondary recovery method wherein a multiple solvent method is employed. The disclosed method was directed to the recovery of hydrocarbons from reservoirs having strata of different permeabilities. The patent disclosed that the solvents would inherently be displaced into the strata in proportion to the permeability of the strata.
All of the above-described techniques are directed to methods for treating the entire formation in some manner in order to recover viscous hydrocarbons; however, the importance of improving the vertical conformance of a formation to recover hydrocarbons apparently was not recognized.
Because of solvent cost, rising energy cost and the rapid depletion of natural energy sources, a process for the recovery of viscous hydrocarbons is therefore desirable to provide a recovery technique which minimizes the loss of solvent while producing high recovery of hydrocarbon.